The invention relates to a projection exposure system having a reticle which operates in reflection.
Projection exposure systems having a reflective reticle have been used in the past, inter alia, together with 1:1 Dyson objectives. These projection exposure systems are described in the following publications:
a) Owen et al, xe2x80x9cxe2x85x9 xcexcm optical lithographyxe2x80x9d J. Vac. Sci. B 10 (1992), pages 3032 to 3036, especially Parts B and C;
b) Pease et al, xe2x80x9cLithography for 0.25 xcexcm and below . . . xe2x80x9d IEEE Symp. VLSI Technology (1992), pages 116 and 117;
c) Jeong et al, xe2x80x9cOptical projection system . . . xe2x80x9d J. Vac. Sci. B 11 (1993), pages 2675 to 2679; and,
d) U.S. Pat. No. 4,964,705.
The incoupling of the illumination takes place via a partially transmitting mirror as shown, for example, in U.S. Pat. No. 4,964,705 (FIGS. 3A and 3B). Beam splitter cubes or beam splitter plates are not provided in these designs.
Reflective reticles are used exclusively in the area of lithography utilizing soft X-rays (EUVL). The beam splitting of illuminating and imaging beam paths is realized by an inclined incidence of the illumination. Beam splitter cubes or beam splitter plates are not used. The objectives are pure mirror objectives having a non-axial symmetrical beam path. The inclined incidence of the illuminating light on the reflective reticle has the disadvantage that the raised mask struts lead to vignetting.
Japanese patent publication 9-017719 discloses a wafer projection exposure system having a reflex LCD as a special reticle. According to FIG. 1 of this publication, a planar beam splitter plate is used to separate the illuminating and imaging beam paths. Illuminating system and projection objective are operated with a field symmetrical to the optical axis. The incoupling of the illuminating light via a beam splitter plate directly ahead of the reticle as shown in Japanese patent publication 9-017719 requires, on the one hand, the corresponding entry intersection distance, and, on the other hand, the passthrough through the planar plate leads to the astigmatic deformation of the illuminating light beam which disturbs the required clean pupil imaging.
U.S. Pat. No. 5,956,174 discloses a catadioptric microscope objective wherein the illuminating light is coupled in via a beam splitter cube between the microscope objective and the tube lens. This type of illumination is conventional in reflected light microscopes. The illuminating field sizes are only in the order of magnitude of 0.5 mm.
Catadioptric systems for wavelengths of 193 nm and 157 nm are known. Catadioptric projection objectives having beam splitter cubes without an intermediate image are shown, for example, in U.S. Pat. Nos. 5,742,436 and 5,880,891 incorporated herein by reference.
Catadioptric projection objectives having a beam splitter cube and an intermediate image are disclosed in U.S. Pat. No. 06/424,471.
Illuminating devices for microlithography are disclosed in U.S. Pat. No. 5,675,401 and U.S. Pat. No. 6,285,443. So-called REMA objectives for imaging a reticle masking device (REMA) into the plane of the reticle are disclosed in U.S. Pat. No. 5,982,558 and U.S. Pat. No. 6,366,410, also incorporated herein by reference. With these objectives, inter alia, the entry pupil of the downstream projection objective is illuminated.
The production of transmission reticles (that is, masks operated in transmission for microlithography) is difficult for deep ultraviolet wavelengths, especially 157 nm, inter alia, because of suitable transmitting carrier materials. The materials CaF2 or MgF2 can be considered. However, reticles made of CaF2 or MgF2 are difficult to process and are therefore very expensive. Furthermore, a reduction of the minimal structural size which can be applied to a semiconductor chip results because of absorption and the thermal expansion of the reticle resulting therefrom when there are multiple illuminations. When possible, materials such as MgF2 are avoided because they are also double refracting.
The alternative are reflective reticles. To reduce the requirements imposed on the reticle, it is advantageous when the projection objective is configured as a reduction objective and the reticle is imaged so as to be demagnified. The reticle can then be provided with larger structures.
In conventional reduction objectives, the use of reflective reticles is not easily possible. The typical entry intersection distance of, for example, 30 mm reduces the illumination at suitable angles of incidence.
It is an object of the invention to provide a projection exposure system having a reduction objective which functions without problem with reflective reticles.
The projection exposure system of the invention is for microlithography and includes: a light source; an illuminating system mounted downstream of the light source for transmitting light from the light source as an illuminating beam along an illuminating beam path; a reflective reticle; a reduction objective defining an imaging beam path and being configured for imaging the reticle onto an object; and, a beam splitter cube mounted in the imaging beam path for mutually superposing the illuminating beam path and the imaging beam path.
According to a feature of the invention, a beam splitter cube functions to superpose the illuminating and imaging beam paths. In this way, numerous objective design concepts for reflective reticles can be adapted as will be shown in the following examples. Erroneous entries by the beam splitter plate are avoided by utilizing a beam splitter cube in lieu of a planar parallel beam splitter plate. The beam splitter plate is operated in passthrough and mounted at 45xc2x0.
According to another feature of the invention, optical elements are provided between the beam splitter cube and the reticle. With these optic elements, it is possible to reduce the angle of incidence of the main beams of the reduction objective on the reticle in such a manner that the incident angle has values between xe2x88x9215 mrad and +15 mrad.
According to still another feature of the invention, the illuminating system is so configured that the illuminating beam path passes over into the imaging beam path with deviations of less than xc2x12.5 mrad. This deviation can be measured in that the angles with respect to the reticle plane are determined for the centroidal rays after the reflection and the deviation to the angles of the corresponding chief rays is computed. The angles of the centroidal rays are dependent upon the emission characteristics of the light source and the design of the illuminating system and the angles of the chief ray are exclusively dependent upon the design of the reduction objective.
According to another feature of the invention, a polarization beam splitter cube is used in order to reduce transmission losses at the beam splitter cube and so that no scattering light is deflected onto the wafer. For an optimal operation, the illuminating light should be linearly polarized to more than 95%. The polarization direction is dependent upon whether the illuminating beam path is intended to be reflected or not at the beam splitter layer. In the case of a reflection, the illuminating light has to be polarized parallel to the beam splitter surface and, in the case of the transmission, the illuminating light has to be polarized perpendicularly to the beam splitter surface.
In other embodiments of the invention, the beam splitter cube functions exclusively for incoupling the illuminating beam path. To be able to more easily integrate the beam splitter cube into the design of the reduction objective, it is advantageous to subdivide the reduction objective into two component objectives with a first intermediate image having an imaging scale of xe2x88x921.0xc2x10.25 and a second image having an imaging scale of xe2x88x920.25xc2x10.15. The beam splitter cube is integrated into the first intermediate image. The second image can be configured to be strictly refractive or catadioptric.
The coupling in of the illuminating beam path with a beam splitter cube is especially advantageous when the beam splitter cube is already a part of the reduction objective. Then, the fourth unused face of the beam splitter cube can be used to couple in the illuminating beam path.
If the design of the catadioptric objective includes a deflecting mirror, then the deflecting mirror can be replaced by a beam splitter cube via which the illuminating light is coupled in.
The design of the catadioptric objective can be configured with or without an intermediate image.
In another embodiment of the invention, a special beam splitter plate is provided in the projection exposure system. This beam splitter plate is operated in pass through in the illuminating beam path and is operated reflectively in the imaging beam path. Here, reflection in air is provided, that is, in the optically thinner medium which can also be a vacuum or a special gas mixture or a gas such as nitrogen or helium. The beam splitter plate is so configured that astigmatic errors because of the plate mounted at an angle can be refractively corrected.
The common inventive concept is that the imaging beam path is held free of disturbances by the beam splitter arrangement and the illuminating beam path is corrected with less requirements directly via the beam splitter arrangement. For a beam splitter cube, only rotationally-symmetrical imaging errors are introduced which can be corrected within the illuminating system via rotationally-symmetrical optical elements such as spherical lenses. In the beam splitter plate according to a feature of the invention, the correction of the illuminating beam path is provided by the special configuration of the side of the beam splitter plate facing toward the illuminating system.
According to still another feature of the invention, the beam splitter plate is provided with a non-planar corrective surface. By mounting the beam splitter plate at an angle, the corrective surface exhibits no rotational symmetry, rather, a simple symmetry with respect to the meridian plane.
The beam splitter plate is configured to have a wedge shape in accordance with another embodiment of the invention for correcting the astigmatism of the lowest order. The use of a beam splitter plate is especially advantageous when it is used in lieu of a deflecting mirror provided in the design of the reduction objective.
The superposition of the illuminating optics and the projection optics make possible the use of reflective reticles especially at operating wavelengths in the range from 100 to 200 nm. In this way, the difficulties are avoided which occur in the manufacture of transmission reticles because of machining of the materials transparent at these wavelengths.